This disclosure concerns a method for the fabrication of a solar cell on an opaque, non-conductive solid substrate, where all of the components of the device can be deposited using a spray-based solution process. Critical elements are found in both the method of deposition as well as in the unique architecture of the cell.
Solution synthesized inorganic nanocrystals are generally composed of an inorganic core surrounded by an organic ligand shell, and each of these components performs a distinct role in device fabrication.
The inorganic core provides electronic function and an opportunity to exploit quantum confinement effects not seen in bulk inorganic materials. For semiconductors, this occurs when the nanocrystal diameter falls below the Bohr exciton radius of the material.
The organic ligand shell stabilizes the core and enables the nanoclusters to dissolve in organic solvents, providing a practical means for the solution processing of inorganic devices.
One area where these materials are currently under intense examination is in the field of photovoltaics (PV), where this combination of electronic tuning via quantum confinement and solution processability hold promise for the fabrication of large area, flexible, and low-cost devices.
Initial approaches to the incorporation of nanocrystals into photovoltaics involved dispersing the material into a conductive polymer matrix. In this configuration, the nanocrystals absorb visible wavelength photons entering the active layer and consequently generate an exciton. Separation of this exciton into an electron and hole is aided by the polymer, serving as an electron transporting layer.
These early designs were plagued by low efficiencies and air sensitivity. In 2005, an all-inorganic design was first reported based on the heterojunction formed between layers of CdTe and CdSe nanorods deposited through a spin coating process and illustrated in FIG. 1. Note that this device contains no polymer in the region between nanorods, relying instead on the nanomaterial itself to generate and transport carriers. In addition to its improved power conversion efficiency of 2-3% under air mass 1.5 global filtered illumination, of particular note for this device is the measured air stability. Subsequent work has focused primarily on alternative material systems such as PbS and PbSe, and the dip coating of nanocrystal films via ligand exchange, with an eye toward harnessing carrier multiplication for high efficiency solar cells.